1. Field of the Invention
The present invention relates to a mold for a metal casting which is used in a continuous metal casting apparatus and, more particularly, to a mold for use in a continuous metal casting having an inner surface of a porous layer which forms a gaseous film on the inner surface of the mold.
2. Description of the Prior Art
According to continuous metal casting apparatus which is conventionally used, a flux is put in the mold together with the smelting (molten steel) and is interposed therebetween, and at the same time the mold is oscillated to prevent baking of the mold, while the smelting is continuously pulled out downwardly from the mold, thereby being cast. However, there are drawbacks such as the addition of the flux adversely effecting the quality of the steel thus produced, while the construction of the apparatus becomes complicated since the mold has to be oscillated. Due to this, a method has been proposed whereby no flux is used but a porous layer is provided on the inner surface of the mold where the smelting is put in and compressed gas is always fed between the smelting and the porous layer through this porous layer of thereby interpose the gaseous film therebetween; while the smelting is pulled out downwardly of the mold for continuous metal casting.
In addition, for the above porous layer, there has been proposed, for example a porous layer which is formed in a manner such that copper powder is put in front of a copper plate and both are pressed to be closely adhered and thereafter sintered, thereby integrally forming a porous layer, wherein this layer is used as the inner wall of the mold. However, since the thermal shrinkage factor of the copper powder is larger than that of the copper plate, it is difficult to sinter both of them as an integral construction for a large sized mold. Moreover, there are many problems such as the occurrence of the cracks in the copper powder portion, occurrence of uneven porosity, and the like. Furthermore, even if they could be integrally constructed, when the copper powder portion is consumed, the copper plate portion also has to be replaced together with the copper powder portion; thus causing the running cost to be increased so as to question the realization of this method.
Also, the lower portion is uniformly formed by the soft copper powder in addition to the porous layer configuration. Due to this, since the (outer) shell of the smelting has already been hardened at the lower portion of the mold due to decrease in surface temperature, the inner surface of the abovementioned porous layer comes into contact with the shell, causing the inner surface of the lower portion of the porous layer to be worn away. Furthermore, an inconvenience occurs in that blowing of the gas becomes worse due to its abrasion.
In recent years, there has been utilized a mold in which an electromagnetic stirring apparatus applies fluid motion to the smelting in the mold to improve the semis quality.
This apparatus applies a principle of the inductive motor, i.e., electromagnetic coils to produce the rotational magnetic fields are arranged around the outer periphery of the mold and fluid motion is applied to the smelting in the mold by the rotational magnetic fields.
However, although the internal quality of the semis can be improved by the electromagnetic stirring apparatus, the improvement in surface quality is insufficient. It has been known that if the stirring speed of the smelting by the electromagnetic stirring is raised, the surface quality will be improved, but entrainment of the flux occurs, so that there is a problem in that the speed cannot be raised as desired.
In addition, in the continuous metal casting method, in order to prevent the surface of the molten metal which was molded in the water-cooled mold from being oxidation polluted by the atmosphere, direct contact with the atmosphere is conventionally prevented by scattering the flux on the surface of the molten metal or by other similar methods. The fused material of this flux enters the boundary between the water-cooled mold and the molten metal and serves as a lubricant. However, this conventional technology requires a supply of flux and the installation of the scattering apparatus, and moreover the prevention of oxidation is insufficient in the gap of the fused slag. Also, the occurrence of entrainment of the flux causes non-metalic inclusion in the semis.
Furthermore, in order to prevent oxidation pollution of the molten metal, a method has been conventionally publicly known whereby the surface of the molten metal is shut off from the outside air by inert gaseous ambience such as argon, nitrogen, etc.
Such an example is shown in Japanese Patent Kokai (Laid-Open) No. 47-33920. However, in such a conventional inert gas shut-off apparatus of this kind, it is generally difficult to effectively utilize the inert gas in spite of its large scale; a large associated gas is lost and a large amount of cost is required; the effect of preventing the pollution is insufficient in spite of the large consumption; and the secondary desirable influence cannot be expected. On the contrary, the cooling speed of the molten metal is increased and interferes and disturbs other work steps. Therefore, it is not always possible to maximize of the primary usefulness and advantage.